thus we have an illuminating plane (moving parallel to the X-axes), which oscillates with a wave-length T′=T/(1−ϰ2ω2){\displaystyle \textstyle {T'=T/\left(1-{\frac {\varkappa ^{2}}{\omega ^{2}}}\right)}} (only different of the second order of T). The propagated oscillation can be written: